1. Field of Endeavor
The present invention relates to the field of power plant technology. It relates to a combined-cycle power plant with electronic decoupling or electronic frequency conversion between the gas turbine and power grid and a steam turbine connected directly to the power grid via a generator, and to a method for operating such a power plant.
2. Brief Description of the Related Art
Large power plants with powers within the range of more than 100 MW, in which a current-generating generator is driven by a gas and/or a steam turbine and feeds the generated electrical power into a power grid having a predetermined grid frequency (e.g. 50 or 60 Hz) usually have a fixed coupling between the (mechanical) speed of the turbine and the grid frequency. In this arrangement, the output of the generator is connected in a frequency-locked manner to the power grid via a system connection whilst it is driven either directly via the turbine (1-shaft plant) or speed-coupled via a mechanical gear unit. Such configurations of power plants are reproduced in a greatly simplified manner in FIGS. 2 and 3. Using gear units, it is only possible to achieve fixed transmission ratios between grid frequency and turbine. However, solutions are also conceivable in which the generator is driven by a utility turbine which can be run at a speed deviating from the actual gas turbine.
FIG. 2 shows, in a greatly simplified representation, a power plant 10′ of the known type which generates power with a gas turbine 12 with a first generator 18 coupled thereto and a steam turbine 24 with a second generator 8 coupled thereto and which is feed their electric power into a power grid 21. The gas turbine 12 and the generator 18 are joined by a common shaft 19 and form a power train 11. In the simplest case, the gas turbine includes a compressor 13 which sucks in and compresses combustion air via an air inlet 16. The compressor 13 can be composed of a number of cascaded part-compressors which operate at a rising pressure level and possibly provide for intermediate cooling of the compressed air. The combustion air compressed in the compressor 13 passes into a combustion chamber 15 into which liquid (e.g. oil) or gaseous (e.g. natural gas) fuel is nozzle-injected via a fuel supply 17 and burnt with consumption of combustion air.
The hot gases emanating from the combustion chamber 15 are expanded in a subsequent turbine 14 whilst performing work and thus drive the compressor 13 and the first generator 18 coupled thereto. The exhaust gas, which is still relatively hot on emerging from the turbine, is sent through a subsequent heat recovery steam generator 23 in order to generate steam for operating a steam turbine 24 in a separate water steam cycle 25. Condenser, feed water pump and other systems of the water-steam cycle 25 are not shown in order to simplify the representation. Such a combination of gas turbine and steam power plant is called a combined-cycle power plant. In this arrangement, the steam turbine 24 can be coupled to the first generator 18 on the side opposite the turbine 14; gas turbine 12, first generator 18 and steam turbine 24 then form a so-called single-shaft power train. However, the steam turbine 24 can also drive a separate second generator 8 on a separate power train 60 as shown in FIG. 2. Various combinations are known for multi-shaft plants. For example, so-called 2-on-1 arrangements are widely used in which a steam turbine 24 on a power train 60 with a second generator 8 is supplied with steam by boilers 23 following two gas turbines 12. In this arrangement, the gas turbines 12 are in each case arranged on a power train 11 having its own first generator 18. Analogously, there are also arrangements in which the steam from three or more boilers 23 following gas turbines 12 is used for driving a steam turbine 24.
In the case of the one-shaft gas turbine of FIG. 2, the speed of the gas turbine 12 has a fixed ratio to the frequency, generated in the first generator 18, of the alternating voltage which must be equal to the grid frequency of the power grid 21. In the case of the large gas turbine units normally used today, with powers of over 100 MW, a speed of the gas turbine of 3600 rpm (e.g. ALSTOM's gas turbine GT24) is allocated to the generator frequency or grid frequency of 60 Hz and a speed of 3000 rpm (e.g. ALSTOM's gas turbine GT26) is allocated to the generator frequency of 50 Hz.
If it is intended to achieve a different ratio between the speed of the gas turbine 12 and the generator or grid frequency, a mechanical gear unit 26 can be inserted in principle between the shaft 19 of the gas turbine 12 and the first generator 18 (power train 11′) in a power plant 10″ according to FIG. 3, which gear unit is normally constructed as a step-down gear unit and thus provides for higher speeds and smaller constructions of the gas turbine 12. Corresponding step-down gear units are also used for operating small steam turbines. However, such mechanical gear units 26 are typically only used for powers of less than 100 MW to 120 MW for reasons of strength. On the other hand, the large powers per gas turbine of over 100 MW and the high efficiencies are mainly achieved with one-shaft machines which rotate comparatively slowly.
This then results in the situation shown in FIG. 1: above about 100 MW useful power, there are individual one-shaft gas turbines which are designed and optimized for a fixed speed of either 3000 rpm (for 50 Hz; GT26) or 3600 rpm (for 60 Hz; GT24) (F. Joos et al., Field experience with the sequential combustion system of the GT24/GT26 gas turbine family, ABB Review no. 5, p. 12-20 (1998)). Above 100 Hz and at powers below 100 MW, almost any alternating-voltage frequencies are possible by configurations with utility turbine or gear unit or by multi-shaft gas turbines (shaded area in FIG. 1). In this, the powers of the gas turbines over frequency follow a curve A whilst the efficiency η follows curve B. Large powers with high efficiencies can thus be mainly achieved at low speeds where, however, only singular solutions are available.
To reduce the manufacturing costs in the singular solutions, it has already been proposed in U.S. Pat. No. 5,520,512 to construct at least parts of the turbines identically in gas turbine plants for different system frequencies. In this arrangement, however, the rigid coupling between the speed of the gas turbine and the grid frequency remains unchanged.
In U.S. Pat. No. 6,628,005 it has been proposed to render a one-shaft plant having a turbine and a generator with a predetermined speed useable for different system frequencies of 50 Hz and 60 Hz by selecting a generator frequency between the two system frequencies of, e.g. 55 Hz and to add or subtract 5 Hz with a frequency differentiator depending on the grid frequency. Here, too rigid coupling is retained.
The rigid coupling between turbine speed and grid frequency for existing plant concepts with existing turbine components resulted in restrictions in the optimization of the steady-state operation. In addition, the transient behavior is also impaired. For example, power dips occur in the turbine or, respectively, thermal and mechanical loading occur during the dynamic control for supporting the grid frequency due to a rise in the gas turbine inlet temperature. Furthermore, rapid transients lead to increased loads.
The optimization in the new design of components or power plants is also restricted due to the rigid coupling between turbine speed and grid frequency. In particular, power plant turbines are limited in the magnitude of their power due to the predetermined coupling to the grid frequency (see curve A in FIG. 1).
From U.S. Pat. No. 5,694,026, a one-shaft-turbine-generator unit without a step-down gear unit is known in which a static frequency converter is arranged between the output of the generator and the power grid, with the aid of which frequency converter the alternating-voltage frequency generated by the generator is converted into the frequency of the power grid. When the gas turbine is started, the generator is used as a motor, said motor being supplied with energy from the power grid via the static frequency converter. The converter consists of a direct-current link circuit formed by an inductance.
From U.S. Pat. No. 6,979,914, a power plant with a one-shaft arrangement of a gas turbine and a generator is known in which a converter is also provided between the generator output and power grid in order to adapt the alternating voltage generated by the generator to the grid frequency. In this case, a direct-voltage link circuit is arranged in the converter.
From the article by L. J. J. Offringa, L. J. J. et al., “A 1600 kW IGBT converter with interface transformer for high speed gas turbine power plants”, Proc. IEEE-IAS Conf. 2000. 4, 8-12 Oct. 2000, Rome, 2000, pp. 2243-2248, a power plant with a rapidly rotating gas turbine (18,000 rpm) and comparatively low output power (1600 kW) is known in which frequency decoupling between generator and power grid is achieved by a converter with direct-voltage link circuit.
In the known power plants with decoupling between generator output and power grid by a frequency converter with direct-current or direct-voltage link circuit, a resultant disadvantage is that the converters entail not inconsiderable power losses which, in power plants with a one-shaft power train and powers of more than 100 MW, cancel out again a part of the improvement in efficiency achieved in this area.